JP2020090609A - Polyethylene-based resin foam particle and polyethylene-based resin foam particle molded body - Google Patents

Abstract

To provide a foam particle that is excellent in antistatic performance even under an environment of low humidity, can suppress the migration of components derived from an antistatic agent into an object to be packaged and has good moldability and to provide a foam particle molded body that comprises the foam particle.SOLUTION: The foam particle has a core layer including (A) a polyethylene-based resin and a covering layer including (B) a polyethylene-based resin and (C) a polymer type antistatic agent and covering the core layer. A straight-chain low-density polyethylene is included in the polyethylene-based resins (A) and (B). A polyether/polyolefin block copolymer is included in the antistatic agent (C). The content of the antistatic agent (C) in the covering layer is 20 to 50 mass%. A melting point Tm[°C] of the polyethylene-based resin (A), a melting point Tm[°C] of the polyethylene-based resin (B) and a melting point Tm[°C] of the antistatic agent (C) satisfy the relationship of 0≤Tm-Tm≤30 and 25≤Tm-Tm≤75.SELECTED DRAWING: Figure 1

Description

The present invention relates to expanded polyethylene resin particles and expanded polyethylene resin resin particles.

Foamed particle moldings made of polyethylene-based resin are widely used as packing materials such as spacers and boxes, and as cushioning packing materials, by taking advantage of their excellent shock absorbing properties. Examples of the articles to be packaged by the cushioning packaging material include precision equipment, electronic equipment, and electronic parts. However, the polyethylene-based resin is easily charged with static electricity, which may lead to adhesion of dust and damage to electronic parts and the like. In order to avoid such a problem, expanded polyethylene resin particles having antistatic performance have been used. For example, Patent Document 1 discloses polyethylene resin foamed particles containing a surfactant as an antistatic agent.

JP, 2017-171779, A

The antistatic effect of the surfactant is exhibited when the surfactant present on the surface of the expanded particle molded article adsorbs moisture in the air. Therefore, the expanded polyethylene resin particles of Patent Document 1 may have difficulty in exhibiting an antistatic effect in an environment with low humidity, such as in winter or in a dry area. Further, in the expanded polyethylene resin particles of Patent Document 1, the components derived from the surfactant may migrate to the object to be packaged, so that the object to be packaged may be contaminated or the antistatic effect may be reduced.

As a method of avoiding various problems when a surfactant is used as the antistatic agent, for example, a method of using a polymer type antistatic agent can be considered. However, when a polymer type antistatic agent is blended as an antistatic agent, there is a problem in that it is difficult to achieve both antistatic performance and fusibility of the expanded particles together. That is, when a high-molecular-type antistatic agent is added in an amount necessary for expressing the desired antistatic performance, the fusibility of the expanded particles decreases, and a good expanded particle molded article can be obtained. difficult. On the other hand, when the amount of the polymeric antistatic agent is reduced to such an extent that a good expanded particle molded article can be obtained, the antistatic performance may be insufficient.

The present invention has been made in view of such a background, excellent antistatic performance even under a low humidity environment, it is possible to suppress the migration of components derived from the antistatic agent to the object to be packaged, moldability is. The present invention provides a good expanded polyethylene resin particle and a molded expanded polyethylene resin particle comprising the expanded particle.

A first aspect of the present invention is a polyethylene-based resin expanded particle according to the following [1] to [6].

[1] A core layer made of a foam containing a polyethylene resin (A),
A polyethylene resin (B) and a polymeric antistatic agent (C), and a coating layer covering the core layer,
The polyethylene resin (A) contains linear low-density polyethylene,
The polyethylene resin (B) contains linear low-density polyethylene,
The polymer type antistatic agent (C) contains a polyether-polyolefin block copolymer,
The content of the polymeric antistatic agent (C) in the coating layer is 20 to 50% by mass,
The melting point Tm _(A) [℃] of the polyethylene resin (A), the melting point Tm _(B) [℃] of the polyethylene resin (B), the melting point Tm of the polymer type antistatic agent (C) _{( C)} [℃]
0≦Tm _(A) −Tm _(B) ≦30 (1)
25≦Tm _(C) −Tm _(A) ≦75 (2)
Polyethylene-based resin expanded particles that satisfy the above relationship.

[2] the melting point Tm _(A) [° C.] of the polyethylene resin (A), the melting point Tm of the polymer type antistatic agent _{(C) (C) [℃} ] and although,
30≦Tm _(C) −Tm _(A) ≦70 (2-1)
The expanded polyethylene resin particles according to [1], which satisfy the relationship of

[3] The polyethylene resin expanded particles according to [1] or [2], wherein the amount of the inorganic fine particles adhered to the surface of the polyethylene resin expanded particles is 100 mg or less per 100 g of the polyethylene resin expanded particles.

[4] the melting point Tm of the polyethylene resin _{(A) (A) [℃} ] and the melting point Tm _(B) of the polyethylene resin (B) [° C.] is higher than both 100 ° C., [1] ~ [ 3) Polyethylene resin expanded particles according to any one of 3).

[5] The polyethylene resin foamed particles according to any one of [1] to [4], wherein the polyethylene resin (A) is a mixture containing linear low-density polyethylene and high-density polyethylene.

[6] The melt flow rate value (I) of the core layer obtained by measurement at a temperature of 190° C. and a load of 2.16 kgf is 0.5 to 5 g/10 min, and the melt flow rate value of the core layer ( The value of the ratio (II)/(I) of I) to the melt flow rate value (II) of the coating layer obtained by measuring at a temperature of 190° C. and a load of 2.16 kgf is 1-5. 1] to polyethylene expanded resin particles according to any one of [5].

Another aspect of the present invention is a polyethylene resin foamed particle molded article obtained by in-mold molding of the polyethylene resin foamed particles of the above aspect,
The foamed polyethylene resin particles have a surface resistivity of less than 1×10 ¹³ Ω at a relative humidity of 12% RH.

The expanded polyethylene resin particles of the present invention (hereinafter referred to as "expanded particles") have excellent antistatic performance even in an environment of low humidity, and suppress migration of components derived from the antistatic agent to the packaged item. It has a good moldability. In addition, a polyethylene resin foamed particle molded article (hereinafter referred to as "foamed particle molded article") comprising the foamed particles has excellent antistatic performance even in an environment of low humidity, and is an antistatic agent for a packaged item. It is possible to suppress the migration of components derived from.

Further, the foamed particles of the present invention, in the manufacturing process, reduce the amount of the dispersant added to the dispersion, or even when the dispersant is not added to the dispersion, blocking the resin particles or the like. It can be suppressed and has good moldability.

FIG. 1 is an explanatory diagram showing a method of calculating the area of a high temperature peak.

The foamed particles include a core layer made of a foam containing the specific polyethylene resin (A), the specific polyethylene resin (B), and the specific polymer type antistatic agent (C), A coating layer that covers the core layer. Hereinafter, the structure of expanded particles in which a core layer such as the expanded particles is covered with a coating layer may be referred to as a sheath-core structure.

The core layer of the expanded beads is made of a foam containing a polyethylene resin (A). The polyethylene resin (A) contains at least linear low-density polyethylene. The polyethylene resin (A) preferably contains linear low density polyethylene as a main component. Specifically, the content of the linear low-density polyethylene in the polyethylene resin (A) is preferably 50% by mass or more, more preferably 60% by mass or more, and 80% by mass or more. Is more preferable.

In the present invention, the linear low-density polyethylene refers to a linear copolymer of ethylene and α-olefin. Specifically, ethylene-butene copolymer, ethylene-hexene copolymer, ethylene-octene copolymer and the like are preferably exemplified.

The linear low-density polyethylene preferably has a density of 910 to 950 kg/m ³ , and more preferably 915 to 940 kg/m ³ . The density of linear low-density polyethylene can be measured by the pycnometer method described in JIS K7112:1999.

The polyethylene resin (A) may contain one or more polymers selected from ethylene homopolymers other than linear low-density polyethylene and copolymers containing ethylene. The content of the ethylene homopolymer other than the linear low-density polyethylene and the ethylene-containing copolymer in the polyethylene resin (A) is preferably 5 to 30% by mass.

Examples of ethylene homopolymers other than linear low-density polyethylene include high-density polyethylene and branched low-density polyethylene. As the copolymer containing ethylene, for example, ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, ethylene-acrylic acid alkyl ester copolymer, ethylene-methacrylic acid copolymer, ethylene-alkyl methacrylate Examples thereof include ester copolymers.

The polyethylene resin (A) preferably contains high-density polyethylene as an ethylene homopolymer other than linear low-density polyethylene. In this case, the hardness of the foamed film of the foamed particles can be increased, and the shrinkage of the foamed particles immediately after foaming or the foamed particle molded body immediately after molding can be further suppressed. In addition, mechanical strength such as compression strength and bending strength of the obtained expanded particle molded article can be further increased. From the viewpoint of sufficiently obtaining these functions and effects, the content of the high-density polyethylene in the polyethylene resin (A) is preferably 5 to 30% by mass, more preferably 6 to 20% by mass, and 7 It is more preferable that the content is -15% by mass.

When the polyethylene resin (A) contains high-density polyethylene as an ethylene homopolymer other than linear low-density polyethylene, the density of the high-density polyethylene is preferably 935 to 965 kg/m ³ , and 940 It is more preferably 960 kg/m ³ . In this case, mechanical strength such as compression strength and bending strength of the obtained expanded bead molded product can be further increased. The density of the high-density polyethylene is measured by the same method as that of the linear low-density polyethylene described above.

Density of the polyethylene resin (A) is preferably 910~950kg / m ^3, more preferably 915~940kg / m ^3.

The melting point Tm _(A) of the polyethylene resin (A) is preferably 90 to 140°C. By setting the melting point Tm _(A) of the polyethylene-based resin (A) within the above range, it is possible to more effectively suppress the phenomenon of resin particles being fused to each other, which is called blocking, in the foaming step. Further, by setting the melting point Tm _(A) of the polyethylene-based resin (A) within the above range, it is possible to further improve the moldability of the expanded beads such as the fusion property and the recovery property. From the viewpoint of further enhancing these functions and effects, the melting point Tm _(A) of the polyethylene resin (A) is more preferably 100 to 135°C, and further preferably 110 to 130°C.

The melting point Tm _(A) of the polyethylene resin (A) can be measured by the plastic transition temperature measuring method defined in JIS K7121:2012. First, the condition of the test piece is adjusted at a heating temperature and a cooling temperature of 10° C./minute according to “when measuring the melting temperature after performing a certain heat treatment”. After that, the heating temperature is set to 10° C./min, the heat flux DSC (that is, differential scanning calorimetry) is performed, and the DSC curve is acquired. The value of the melting point Tm _(A) can be determined based on the obtained DSC curve. When a plurality of endothermic peaks appear on the DSC curve, the apex of the endothermic peak having the highest apex is taken as the melting point with reference to the high temperature side baseline.

The heat of fusion of the polyethylene resin (A) is preferably 80 to 140 J/g. By setting the heat of fusion of the polyethylene resin (A) within the above range, it is possible to further suppress the decrease in the closed cell rate of the expanded particles. Further, by setting the heat of fusion of the polyethylene resin (A) within the above range, it is possible to further suppress the shrinkage of the expanded particles immediately after expansion and the expanded particle molded product immediately after molding. From the viewpoint of further enhancing these effects, the heat of fusion of the polyethylene resin (A) is more preferably 90 to 130 J/g.

The heat of fusion of the polyethylene resin (A) is measured using a heat flux differential scanning calorimeter based on JIS K7122:2012. First, the condition of the test piece is adjusted by setting the heating temperature and the cooling temperature to 10° C./min according to “when measuring heat of fusion after performing constant heat treatment”. After that, the heating temperature is set to 10° C./min, the heat flux DSC (that is, differential scanning calorimetry) is performed, and the DSC curve is acquired. The value of the heat of fusion can be determined based on the obtained DSC curve. When a plurality of melting peaks appear on the DSC curve, the total area of the plurality of melting peaks is taken as the heat of fusion.

The melt flow rate (MFR(A)) of the polyethylene resin (A) is preferably 0.5 to 5.0 g/10 min. By setting the value of MFR(A) within the above range, it is possible to further reduce the apparent density of the expanded particles while maintaining a high closed cell rate. From the viewpoint of further enhancing such action and effect, the value of MFR(A) is more preferably 0.5 to 4.0 g/10 min, and further preferably 1.0 to 3.0 g/10 min.

The melt flow rate of the polyethylene-based resin (A) is a value measured under the conditions of a temperature of 190° C. and a load of 2.16 kg in accordance with JIS K7210-1:2014.

In addition to the polyethylene resin (A), the core layer may contain additives such as a cell regulator, a catalyst neutralizing agent, a lubricant and a crystal nucleating agent. The content of the additive in the core layer is, for example, preferably 15% by mass or less, more preferably 10% by mass or less, further preferably 5% by mass or less, and 1% by mass or less. It is particularly preferable that

The core layer contains, in addition to the polyethylene resin (A), other resins and materials such as elastomers other than the polyethylene resin (A) within a range not impairing the objects and effects of the present invention. Good. Examples of the resin other than the polyethylene resin (A) include thermoplastic resins such as polypropylene resin, polystyrene resin, polybutene resin, polyamide resin, and polyester resin. Examples of the elastomer other than the polyethylene resin (A) include olefin thermoplastic elastomer and styrene thermoplastic elastomer. The content of resins and elastomers other than the polyethylene resin (A) contained in the core layer is preferably 20 mass% or less, more preferably 10 mass% or less, and 0 mass%, that is, It is more preferable that the resin other than the polyethylene resin (A) and the elastomer are not contained.

From the viewpoint of the mechanical strength of the expanded particle molded article, the core layer preferably contains substantially no antistatic agent, more preferably no antistatic agent. The term "substantially free" means that the content of the antistatic agent in the core layer is 3% by mass or less (including 0% by mass), preferably 1% by mass or less (including 0% by mass). ) Means.

The melt flow rate (I) of the core layer is preferably 0.5 to 5.0 g/10 min, more preferably 0.7 to 4.5 g/10 min, and 1.0 to 4.0 g/min. More preferably, it is 10 minutes. By setting the value of the melt flow rate (I) within the above range, it is possible to lower the apparent density of the expanded particles while maintaining a high closed cell rate. The melt flow rate (I) of the core layer is measured by the same method as the melt flow rate of the polyethylene resin (A) described above. The melt flow rate (I) of the core layer means a melt flow rate of a material forming the core layer, that is, a mixture containing the polyethylene resin (A) and the above additives.

The surface of the core layer is covered with a coating layer containing a polyethylene resin (B) and a polymeric antistatic agent (C). The polyethylene resin (B) contains at least linear low-density polyethylene. The polyethylene resin (B) preferably contains a linear low-density polyethylene resin as a main component. Specifically, the content of linear low-density polyethylene in the polyethylene resin (B) is preferably 50% by mass or more, more preferably 80% by mass or more, and linear low-density polyethylene. More preferably, it consists of only.

The polyethylene resin (B) may contain one or more polymers selected from homopolymers of ethylene other than linear low-density polyethylene and copolymers containing ethylene. Examples of the homopolymer and the copolymer containing ethylene that can be contained in the polyethylene resin (B) are the same as those of the polyethylene resin (A). Further, the content is preferably about 20% by mass or less.

The melting point Tm _(B) of the polyethylene resin (B) is preferably 80 to 130°C. By setting the melting point Tm _(B) of the polyethylene resin (B) within the above range, blocking can be more effectively suppressed in the foaming step. Further, by setting the melting point Tm _(B) of the polyethylene-based resin (B) within the above range, it is possible to further improve the moldability of the expanded beads such as the fusion property and the recovery property. From the viewpoint of further enhancing these effects, the melting point Tm _(B) of the polyethylene resin (B) is more preferably 100 to 128°C, further preferably 105 to 125°C. The melting point Tm _(B) of the polyethylene resin ( _B) is measured by the same method as the melting point Tm _{(A) of} the polyethylene resin (A) described above.

The heat of fusion of the polyethylene resin (B) is preferably 50 to 150 J/g. The expanded beads in which the heat of fusion of the polyethylene resin (B) is within the above range can form a good expanded beads molded product even under conditions of low molding pressure. Further, by setting the heat of fusion of the polyethylene resin (B) within the above range, it is possible to further suppress the shrinkage of the expanded particle molded article after molding. From the viewpoint of further enhancing these effects, the heat of fusion of the polyethylene resin (B) is more preferably 70 to 130 J/g. The amount of heat of fusion of the polyethylene resin (B) is measured by the same method as the amount of heat of fusion of the polyethylene resin (A) described above.

The melt flow rate (MFR(B)) of the polyethylene resin (B) is preferably 0.5 to 5.0 g/10 min, from the viewpoint of the production stability of the expanded particles having a sheath-core structure, and is 0.7. ˜4.5 g/10 min is more preferred, and 1.0 to 4.0 g/10 min is even more preferred. The melt flow rate of the polyethylene resin (B) is measured by the same method as the melt flow rate of the polyethylene resin (A).

The coating layer in the foamed particles contains a polymeric antistatic agent (C). The content of the polymeric antistatic agent (C) in the coating layer is 20 to 50% by mass. If the content of the polymeric antistatic agent (C) is less than 20% by mass, the desired antistatic performance may not be obtained. When the content of the polymeric antistatic agent (C) exceeds 50% by mass, the effect of improving the antistatic performance becomes insufficient even if the content of the polymeric antistatic agent (C) is further increased. However, it may be difficult to obtain the antistatic performance suitable for the blended amount. Further, in this case, there is a possibility that the moldability may be deteriorated, such as the deterioration of the fused property of the expanded particles, the occurrence of the sink of the expanded particle molded product, the deterioration of the surface property of the expanded particle molded product, and the like. From the viewpoint of more reliably avoiding these problems and enhancing the antistatic performance, the content of the polymeric antistatic agent (C) in the coating layer is more preferably 23 to 40% by mass, and preferably 25 to 35. It is more preferable that the content is% by mass.

The polymer type antistatic agent (C) contains a polyether-polyolefin block copolymer. The polymeric antistatic agent (C) is preferably composed of a polyether-polyolefin block copolymer. By using a polyether-polyolefin block copolymer as the high molecular type antistatic agent (C), the antistatic performance of the expanded particles is enhanced, and at the same time, a component derived from the antistatic agent from the expanded particle molded product to the packaged object. Can be suppressed. Further, unlike other polymer type antistatic agents, the polyether-polyolefin block copolymer can be added to the coating layer without impairing the performance of the expanded particles. The reason for this is not clear, but it is considered that the polyether-polyolefin block copolymer is excellent in compatibility with the polyethylene resin (B) contained in the coating layer.

Examples of the polyether-polyolefin block copolymer include a copolymer in which polyethylene oxide blocks and polyolefin blocks are alternately bonded. As the polymer type antistatic agent (C), one type of copolymer selected from polyether-polyolefin block copolymers may be used alone, or two or more types of copolymers may be used in combination. May be. Examples of commercially available polyether-polyolefin block copolymers include trade names “Pelestat (registered trademark) 230” and “Perectron (registered trademark) UC” manufactured by Sanyo Chemical Industries, Ltd.

The melting point Tm _(C) of the polymeric antistatic agent (C) is preferably 90 to 180° _C . By setting the melting point Tm _(C) of the polymer type antistatic agent (C) within the above range, the antistatic performance of the expanded beads is further enhanced, and at the same time, it is derived from the antistatic agent from the expanded particle molded product to the packaged object. It is possible to further suppress the migration of the constituents. Further, by setting the melting point Tm _(C) of the polymeric antistatic agent (C) within the above range, it is possible to further improve the fusibility of the expanded particles. From the viewpoint of further enhancing these effects, the melting point Tm _(C) of the polymer type antistatic agent (C) is more preferably 115 to 178°C, further preferably 130 to 175°C, and 140 It is particularly preferable that the temperature is ˜170° C.

The melting point Tm _(C) of the polymer type antistatic agent (C) can be measured according to the method described in JIS K7121:2012. Specifically, the condition of the test piece is adjusted at a heating temperature and a cooling temperature of 10° C./min in accordance with “when measuring the melting temperature after performing a certain heat treatment”. After that, the heating temperature is set to 10° C./min, the heat flux DSC (that is, differential scanning calorimetry) is performed, and the DSC curve is acquired. The temperature at the apex of the melting peak appearing on the DSC curve is defined as the melting point Tm _(C) . When two or more melting peaks appear, the melting point Tm _(C) is the temperature at the apex of the melting peak located on the highest temperature side.

The melt flow rate (MFR(C)) of the polymer type antistatic agent (C) is preferably 8 to 20 g/10 min. When the MFR (C) is within the above range, the antistatic performance of the expanded beads can be further enhanced, and at the same time, the migration of the low molecular weight component from the expanded beads molded article to the packaged object can be further suppressed. Further, when the MFR (C) is within the above range, the fusibility of the expanded particles can be further improved. From the viewpoint of further enhancing these effects, the MFR(C) is more preferably 10 to 15 g/10 min, further preferably 11 to 14 g/10 min.

Further, the melt flow rate of the polymer type antistatic agent (C) is preferably higher than the melt flow rate of the polyethylene resin (B). In this case, since the antistatic agent can be more uniformly dispersed in the polyethylene resin (B), the antistatic performance can be further improved and the variation in the antistatic performance can be further suppressed. From the viewpoint of further enhancing these actions and effects, the melt flow rate of the polymeric antistatic agent (C) is preferably 3 g/10 min or higher than the melt flow rate of the polyethylene resin (B), and preferably 5 g/10 min or more. It is more preferable that it is high, and it is even more preferable that it is 8 g/10 min or more. The upper limit of the difference between the melt flow rate of the polymeric antistatic agent (C) and the melt flow rate of the polyethylene resin (B) is about 20 g/10 min.

The melt flow rate of the polymer type antistatic agent (C) is a value measured under the conditions of a temperature of 190° C. and a load of 2.16 kg in accordance with JIS K7210-1:2014.

The coating layer may contain additives such as a catalyst neutralizing agent, a lubricant, and a crystal nucleating agent, in addition to the polyethylene resin (B) and the polymeric antistatic agent (C). The content of the additive in the coating layer is, for example, preferably 15% by mass or less, more preferably 10% by mass or less, further preferably 5% by mass or less, and 1% by mass or less. It is particularly preferable that

In addition to the polyethylene-based resin (B) and the polymeric antistatic agent (C), the coating layer may include the polyethylene-based resin (B) and the polymeric antistatic agent as long as the objects and effects of the present invention are not impaired. Resins other than the inhibitor (C), elastomers, etc. may be contained. Examples of these resins and elastomers are the same as those of the core layer described above. The content of the resin or elastomer other than the polyethylene resin (B) and the polymeric antistatic agent (C) in the coating layer is preferably less than 10 mass%, that is, the polyethylene resin. It is more preferable to contain no resin or elastomer other than (B) and the polymeric antistatic agent (C).

The melt flow rate (II) of the coating layer is preferably 0.5 to 20 g/10 min, more preferably 0.7 to 15 g/10 min, and further preferably 1.0 to 10 g/10 min. preferable. By setting the value of MFR(II) within the above range, the core layer can be more uniformly coated with the coating layer. Thereby, the antistatic performance of the expanded particles can be further improved. The melt flow rate (II) of the coating layer is measured by the same method as the melt flow rate of the polyethylene resin (A) described above. The melt flow rate (II) of the coating layer is the melt flow rate of a material that constitutes the coating layer, that is, a mixture containing a polyethylene resin (B), a polymeric antistatic agent (C), the additive, and the like. Means late.

In the above expanded particles, the melting point Tm of the polyethylene resin (A) and _(A) [° C.], and the melting point Tm of the polyethylene resin _{(B) (B) [℃} ] is
0≦Tm _(A) −Tm _(B) ≦30 (1)
Are happy with the relationship.

When the value of Tm _(A) -Tm _(B) is less than 0, the fusible property of the foamed particles becomes low, and the mechanical strength such as the tensile strength and the bending strength of the foamed particle molded body is lowered. There is a risk. If the value of Tm _(A) -Tm _(B) is larger than 30, blocking may occur during foaming, that is, mutual sticking of resin particles may easily occur.

From the viewpoint of more reliably avoiding deterioration of mechanical strength and blocking, the melting point Tm _(A) [°C] of the polyethylene resin (A) and the melting point Tm _(B) [°C] of the polyethylene resin (B) are ,
0.5≦Tm _(A) −Tm _(B) ≦25 (1-2)
It is preferable that the relationship of
1≦Tm _(A) −Tm _(B) ≦20 (1-3)
It is more preferable to satisfy the relationship of.

Melting point Tm of the polyethylene resin _{(A) (A) [℃} ] and polyethylene melting point Tm of the resin _{(B) (B) [℃} ] is preferably higher than both 100 ° C.. In this case, blocking can be suppressed more effectively in the foaming step. Further, when the melting point Tm of the polyethylene resin _{(A) (A) [℃} ] and the melting point Tm of the polyethylene resin _{(B) (B) [℃} ] is higher than both 100 ° C., the fusion of the expanded beads The sex can be enhanced. Further, if the melting point Tm of the polyethylene resin _{(A) (A) [℃} ] and the melting point Tm of the polyethylene resin _{(B) (B) [℃} ] is higher than both 100 ° C., the foamed bead molded article The mechanical strength can be further improved. From the viewpoint of enhancing these effects, it is more preferable that both Tm _(A) [°C] and Tm _(B) are higher than 105°C.

Furthermore, the melting point Tm of the polyethylene resin (A) in the expanded particles _(A) [° C.], polymer type antistatic agent (C) and the melting point of the melting point Tm of the _(C) [° C.] is
25≦Tm _(C) −Tm _(A) ≦75 (2)
Are happy with the relationship.

When the value of Tm _(C) -Tm _(A) is less than 25, the fusibility of the expanded particles may be deteriorated. Particularly, the melting point Tm _{(C) of the} polymeric antistatic agent (C) contained in the coating layer is relatively low, and the difference from the melting point Tm _(A) of the polyethylene resin (A) contained in the core layer is too small. In this case, the fusibility of the expanded particles tends to decrease. When the value of Tm _(C) -Tm _(A) is larger than 75, the antistatic performance of the expanded particle molded article may be deteriorated and the foamability of the expanded particles may be decreased.

From the viewpoint of avoiding these problems more reliably, the melting point Tm of the polyethylene resin (A) and _(A) [° C.], a melting point Tm _(C) [° C.] of the polymeric antistatic agent (C) ,
30≦Tm _(C) −Tm _(A) ≦70 (2-1)
It is preferable that the relationship of
32≦Tm _(C) −Tm _(A) ≦60 (2-2)
It is more preferable that the relationship of
35≦Tm _(C) −Tm _(A) ≦50 (2-3)
It is more preferable that the relationship of is satisfied.

As described above, when the conventional polyethylene-based resin foamed particles are blended with the polymer-type antistatic agent in an amount necessary for expressing the desired antistatic performance, the fusibleness of the foamed particles is lowered. However, there is a problem that it is difficult to obtain a good expanded particle molded body. Further, in the polyethylene-based resin expanded particles, the core layer has a sheath-core structure covered with a coating layer, and even when a polymer-type antistatic agent is added to the coating layer, the fusibility of the expanded particles to each other, It was difficult to achieve both antistatic performance at the same time.

The foamed particles of the present invention have a melting point Tm _(A) [°C] of the polyethylene resin (A) constituting the core layer and a melting point Tm _(C) [°C] of the melting point of the polymer type antistatic agent (C). However, by satisfying the above specific relationship, excellent antistatic performance even under a low humidity environment, it is possible to suppress the migration of components derived from the antistatic agent to the object to be packaged, good moldability Have

In the DSC curve obtained by heat flux DSC, the core layer of the foamed particles is located on the higher temperature side than the apex of the endothermic peak peculiar to the polyethylene resin (A) contained in the core layer (hereinafter, referred to as “specific peak”). It preferably has a crystal structure in which one or more endothermic peaks (hereinafter referred to as “high temperature peaks”) appear. In this case, the closed cell ratio of the expanded beads can be further increased, and the molding conditions for molding the expanded beads molded product can be selected from a wide range. From this viewpoint, the endothermic amount at the high temperature peak (hereinafter, referred to as “high temperature peak calorific value”) is preferably 5 to 50 J/g, and more preferably 10 to 45 J/g.

The high temperature peak calorific value of the expanded beads can be calculated, for example, by the following method. First, heat flux DSC is performed using 1 to 3 mg of expanded particles excluding the coating layer to obtain a DSC curve. At this time, the measurement start temperature is 10 to 40° C., the measurement end temperature is 220° C., and the temperature rising rate is 10° C./min. When the expanded particles have a high temperature peak, the DSC curve shows a unique peak ΔH1 and a high temperature peak ΔH2 having a peak on the higher temperature side than the peak of the unique peak ΔH1, as shown in FIG.

Next, a straight line L1 connecting a point α corresponding to 80° C. on the DSC curve and a point β corresponding to the melting end temperature T of the expanded particles is drawn. The melting end temperature T is the end point on the high temperature side of the high temperature peak ΔH2, that is, the intersection of the high temperature peak ΔH2 and the baseline on the high temperature side of the high temperature peak ΔH2 in the DSC curve.

After the straight line L1 is drawn, a straight line L2 passing through the maximum point γ existing between the characteristic peak ΔH1 and the high temperature peak ΔH2 and parallel to the vertical axis of the graph is drawn. The straight line L2 divides the characteristic peak ΔH1 and the high temperature peak ΔH2. The heat absorption amount of the high temperature peak ΔH2 can be calculated based on the area of the portion forming the high temperature peak ΔH2 in the DSC curve, the area surrounded by the straight line L1 and the straight line L2.

When the expanded particles are once cooled and the DSC curve is obtained again after the DSC curve is obtained by the method described above, only the unique peak ΔH1 appears in the DSC curve and the high temperature peak ΔH2 disappears from the DSC curve.

In the expanded particles, the melt flow rate value (I) of the core layer is 0.5 to 5 g/10 min, and the ratio (II)/( to the melt flow rate value (II) of the coating layer is (II)/( The value of I) is preferably 1-5. In this case, the core layer can be more uniformly coated with the coating layer. As a result, the antistatic performance of the expanded beads can be further enhanced. From the viewpoint of further enhancing the antistatic performance of the foamed particles, the value of the melt flow rate ratio (II)/(I) is more preferably 1.5 to 4, and further preferably 2 to 3.

The mass ratio (mass %) of the core layer and the coating layer in the foamed particles is preferably core layer:coating layer=99.5:0.5 to 80:20. In this case, it is possible to further improve the fusion property of the expanded particles and further improve the antistatic performance of the expanded particle molded body. From the viewpoint of further enhancing these effects, the mass ratio (mass %) of the core layer and the coating layer is more preferably core layer:coating layer=98:2 to 80:20, and 96:4 to 90. More preferably, it is: 10.

Next, a method for producing the expanded beads will be described.

The polyethylene resin foam particles, for example,
A polyethylene resin (A) is included in the core layer, and a polyethylene resin (B) and a polymeric antistatic agent (C) are included in the coating layer to cover the core layer. ) Contains linear low-density polyethylene, the polyethylene resin (B) contains linear low-density polyethylene, and the polymeric antistatic agent (C) contains polyether. A polyolefin block copolymer is contained, the content of the polymeric antistatic agent (C) in the coating layer is 20 to 50% by mass, and the melting point Tm _{(of the} polyethylene resin (A) _{( a)} and [° C.], a melting point Tm _(B) [° C.] of the polyethylene resin (B), the melting point Tm of the polymer type antistatic agent _{(C) (C) [℃} ] and is represented by the following formula ( The polyethylene-based resin particles satisfying the relationships 1) to (2) are prepared,
The polyethylene resin particles dispersed in an aqueous medium in a closed container are impregnated with a foaming agent under heating,
The polyethylene-based resin particles containing the foaming agent are discharged together with an aqueous medium from a closed container to be foamed.
0≦Tm _(A) −Tm _(B) ≦30 (1)
25≦Tm _(C) −Tm _(A) ≦75 (2)

The polyethylene resin particles are produced, for example, by the following extrusion/cutting steps. First, a strand in which the periphery of the core layer is covered with the coating layer is produced by extrusion molding. After that, the strand is cut into a desired size by a pelletizer or the like to obtain resin particles having an unfoamed core layer and a coating layer.

In the foaming step, the resin particles are placed in a closed container and dispersed in an aqueous dispersion medium such as water. At this time, if necessary, a dispersant for dispersing the resin particles is added to the dispersion medium in the closed container.

Examples of the dispersant include inorganic fine particles such as aluminum oxide, tricalcium phosphate, magnesium pyrophosphate, zinc oxide, kaolin, and mica, and surfactants such as sodium alkylbenzenesulfonate, sodium dodecylbenzenesulfonate, and sodium alkanesulfonate. Can be used. As the dispersant, one kind selected from these inorganic fine particles and a surfactant may be used alone, or two or more kinds may be used in combination.

After sealing the airtight container, a foaming agent is added to the container, and pressure and heating are performed while stirring to impregnate the core layer of the resin particles with the foaming agent. A sheath-core structure including a core layer in a foamed state and a coating layer covering the core layer by discharging the content of the closed container under atmospheric pressure at a foaming temperature after the foaming agent is sufficiently impregnated in the resin particles. It is possible to obtain foamed particles of

In the present invention, the foaming temperature T _ex [° C.] and the melting point Tm _(A) [° C.] of the polyethylene resin (A) are
−5≦Tm _(A) −T _ex ≦10 (3)
It is preferable to satisfy the relationship. In this case, blocking can be suppressed more effectively in the foaming step. From the viewpoint of further increasing the blocking suppressing effect, the foaming temperature T _ex [° C.] in the foaming step and the melting point Tm _(A) [° C.] of the polyethylene resin (A) are
-3≦Tm _(A) −T _ex ≦8 (3-1)
It is more preferable that the relationship of
0≦Tm _(A) −T _ex ≦5 (3-2)
It is more preferable that the relationship of is satisfied.

As the foaming agent, for example, hydrocarbons such as butane, pentane and hexane, halogenated hydrocarbons such as trichlorofluoromethane, dichlorofluoromethane and tetrachlorodifluoroethane, carbon dioxide, nitrogen, inorganic gases such as air, water and the like are used. can do. As the foaming agent, these substances may be used alone, or two or more substances may be used in combination.

As the foaming agent, it is preferable to use an inorganic physical foaming agent containing an inorganic gas such as carbon dioxide, nitrogen or air, and it is more preferable to use an inorganic physical foaming agent containing carbon dioxide. The content of the inorganic gas in the inorganic physical foaming agent is preferably 50 mol% or more, more preferably 70 mol% or more, and further preferably 90 mol% or more in molar ratio.

When an organic physical foaming agent such as hydrocarbon or halogenated hydrocarbon is used as the foaming agent, n-butane, 2-methylpropane, n-pentane, 2- Preference is given to using methylbutane as blowing agent.

In the above-mentioned manufacturing method, the apparent density of the expanded beads can be further reduced by performing a two-stage expansion process on the expanded beads after the expansion process, if necessary.

As the two-step foaming step, for example, the following method can be adopted, but a known method other than this method can be adopted. First, the expanded particles are cured under atmospheric pressure. Next, the expanded particles are transferred to a closed container, and the inside pressure of the expanded particles is increased by pressurizing the inside of the closed container with compressed air or the like. Then, the density of the expanded beads can be lowered by taking out the expanded beads from the closed container and heating them with steam or hot air to expand again.

The apparent density of the expanded beads is preferably 15~300kg / m ^3, more preferably from 25~250kg / m ^3, more preferably a 30~220kg / m ^3. By setting the apparent density of the expanded particles within the above range, it is possible to mold an expanded particle molded body that is lighter in weight and is superior in mechanical strength and the like.

The apparent density of the expanded beads is obtained as follows. First, a foamed particle group consisting of a large number of foamed particles is left for 2 days under the conditions of relative humidity 50%, temperature 23° C. and 1 atm. Then, a graduated cylinder containing water at a temperature of 23° C. is prepared, and the pre-weighed expanded particle group is immersed in the graduated cylinder using a tool such as a wire net. Then, taking into account the volume of tools such as wire netting, the volume of the expanded particle group read from the water level rise is measured. The apparent density (kg/m ³ ) of the expanded particles is obtained by dividing the mass (kg) of the expanded particle group placed in the graduated cylinder by the volume (m ³ ).

The closed cell ratio of the expanded beads of the present invention is preferably 70% or more, more preferably 75% or more, and further preferably 80% or more. When the closed cell ratio of the expanded beads is within the above range, the mechanical strength such as the compressive strength and bending strength of the obtained expanded particle molded product can be further increased. The closed cell ratio is the ratio of the volume of the closed cells to the volume of all the bubbles in the expanded particles, and can be obtained by using an air-comparison hydrometer based on ASTM-D2856-70.

The expanded particle molded body is formed by molding the expanded particles in a mold. As the in-mold molding method, for example, the following method can be adopted, but other known in-mold molding methods can be adopted. First, a mold designed to have a shape corresponding to a polyethylene resin expanded particle molded article to be obtained is prepared. After filling the foamed particles in the mold, steam (steam) is supplied into the mold to heat the expanded particles. At this time, the coating layers of the adjacent foamed particles are fused to each other, and the foamed particles are secondarily foamed to fill the gaps between the foamed particles, so that a large number of the foamed particles filled in the mold are integrated. Turn into. Then, the mold is cooled, and the contents are taken out from the mold to obtain a polyethylene-based resin expanded particle molded body.

Immediately after the in-mold molding, the expanded particle molded body contains water derived from steam introduced into the mold during molding. In order to remove this water from the expanded bead molded product, a curing step of heating the expanded bead molded product using an oven or the like may be performed, if necessary.

The surface resistivity values of the foamed particle molded product at relative humidity of 50% and 12% are both preferably less than 1×10 ¹³ Ω. When the surface resistivity is less than 1×10 ¹³ Ω, the foamed particle molded article has excellent antistatic performance and less dust or dirt. Furthermore, the foamed particle molded body has a surface resistivity value within the above range even under an environment of a low relative humidity of 12%, and is excellent in antistatic performance. From the above viewpoint, it is more preferable that the surface resistivity values at relative humidity of 50% and 12% are both less than 1×10 ¹² Ω.

The surface resistivity of the foamed particle molded product is a value measured according to JIS K6271:2001. Specifically, first, three test pieces are cut out from the central portion of the expanded particle molded body. The dimensions of the test piece are 100 mm length×100 mm width×10 mm thickness. After leaving these test pieces in an atmosphere of 23° C. and 50% relative humidity for 24 hours, 30 seconds after applying an applied voltage of 500 V to each test piece in an atmosphere of 23° C. and 50% relative humidity Measure the current value of. The surface resistivity of each test piece is calculated based on this current value. Then, the arithmetic mean value of the surface resistivity obtained for each test piece is defined as the surface resistivity of the expanded particle molded body. As a device for measuring the surface resistivity, for example, "HIRESTA MCP-HT450" manufactured by Mitsubishi Chemical Analytech Co., Ltd. can be used.

Apparent density of the foamed bead molded article is preferably 10~185kg / m ^3, more preferably from 15~155kg / m ^3, and still more preferably from 20~135kg / m ^3. In this case, it is possible to easily reduce the weight of the expanded particle molded article and further improve the characteristics such as mechanical strength. The apparent density of the expanded bead molded product is obtained as follows. First, the foamed particle molded body is allowed to stand for two days under the conditions of relative humidity of 50%, temperature of 23° C. and 1 atm. Next, a scaled container containing water at a temperature of 23° C. is prepared, and the pre-weighed foamed particle molded body is immersed in the container using a tool such as a wire net. Then, in consideration of the volume of the tool such as a wire net, the volume of the expanded particle molded body which is read from the water level rise is measured. The apparent density (kg/m ³ ) of the foamed particle molded body is obtained by dividing the mass (kg) of the foamed particle molded body placed in the container by the volume (m ³ ).

As described so far, the expanded beads of the above aspect have excellent moldability. Further, a foamed particle molded product obtained by in-molding the foamed particles of the above-mentioned embodiment has excellent antistatic performance even under a low humidity environment, and an antistatic agent from the foamed particle molded product to a packaged object. It is possible to suppress the migration of components derived from.

Furthermore, as a result of diligent studies, the present inventors have found that the expanded particles of the above-described embodiment reduce the amount of the dispersant added to the dispersion in the manufacturing process of the expanded particles, or add the dispersant to the dispersion. It has been found that even when it is not added, it is possible to suppress blocking and the like, and it is possible to obtain the effect of forming expanded particles having good moldability.

As described above, the process for producing expanded polyethylene resin particles has the following expanding process. That is, resin particles are dispersed together with a dispersant in an aqueous medium in a closed container such as an autoclave, and then the dispersed resin particles are impregnated with a foaming agent. Then, the hermetically sealed container is opened and the resin particles are discharged under atmospheric pressure, whereby the resin particles are foamed to produce expanded particles.

In producing conventional polyolefin resin foamed particles, in the foaming step, a dispersant is added to the dispersion liquid in the sealed container in order to suppress blocking in the sealed container and improve the dispersibility of the resin particles. Has been done. Inorganic fine particles such as kaolin are often used as the dispersant. In the foaming step, if it is attempted to reduce the amount of the dispersant added to the dispersion, or to produce the expanded particles without adding the dispersant to the dispersion, the resin particles are fused to each other, which is excellent. It may not be possible to produce expanded particles.

Further, when the amount of the dispersant used is large, a large amount of the inorganic fine particles will adhere to the surface of the expanded particles obtained by expanding the resin particles. However, if the amount of the inorganic fine particles adhered to the surface of the expanded beads is excessively large, the fusibility of the expanded beads during in-mold molding may decrease. Further, from the viewpoint of manufacturing cost and environmental load, there is a demand for expanded particles that can be manufactured without reducing the amount of dispersant used or adding it.

To address this problem, the foamed particles include a core layer made of a foam containing a specific polyethylene resin (A), a specific polyethylene resin (B), and a specific polymer type antistatic agent (C). By including a coating layer that covers the core layer, in the manufacturing process, the amount of the dispersant added to the dispersion is reduced, or even when the dispersant is not added to the dispersion, Good foamed particles can be produced without blocking or the like. Further, since the amount of the inorganic fine particles attached to the surface of the expanded beads can be reduced, the expanded beads have a good fusion property during in-mold molding. Further, manufacturing cost and environmental load can be reduced.

From the viewpoint of more surely avoiding the above-mentioned problems caused by the inorganic fine particles, the amount of the inorganic fine particles adhering to the surface of the expanded particles is preferably 200 mg or less per 100 g of the expanded particles. In this case, it is possible to further improve the fusion property between the expanded particles. Therefore, for example, even when performing in-mold molding with a mold having a cavity having a complicated shape, the foamed particles can be more easily fused to each other. As a result, it is possible to more easily obtain a foamed particle molded body having a complicated shape.

From the viewpoint of further improving the fusion property of the expanded beads, it is more preferable that the amount of the inorganic fine particles attached to the surface of the expanded particles is small. From this point of view, the amount of the inorganic fine particles attached to the surface of the expanded beads per 100 g of the expanded beads is more preferably 150 mg or less, further preferably 120 mg or less, particularly preferably 100 mg or less, and Most preferably, the inorganic fine particles are not attached to the surface of the particles.

The amount of the inorganic fine particles attached can be determined as follows. First, the expanded particles are dried in an oven at 60° C. for 24 hours, and then the expanded particles taken out of the oven are immediately left for 72 hours in a room set to a temperature of 23° C. and a relative humidity of 50%. Next, in a room set under the same conditions, about 100 g of the expanded particles are accurately weighed to the third decimal place, and then the value obtained by rounding off the third decimal place is taken as the mass of the expanded particles to which the inorganic fine particles are attached.

Next, the total amount of the expanded particles after measuring the mass is washed to completely wash out the inorganic fine particles adhering to the surface of the expanded particles. Specifically, it is first dipped in 5 L of a 1N hydrochloric acid aqueous solution, and then dipped in 5 L of ion-exchanged water to wash off the hydrochloric acid. Further, it is dipped in 5 L of a 1 N sodium hydroxide aqueous solution and then dipped in 5 L of ion-exchanged water to wash off sodium hydroxide. After that, it was dipped in 5 L of an aqueous solution obtained by diluting Aqualic (registered trademark) DL522 (30% aqueous solution of polyacrylic acid) manufactured by Nippon Shokubai Co., Ltd. 5 times, and then immersed in 5 L of ion-exchanged water to sodium polyacrylate. Wash off. After repeating the washing operation consisting of these processes twice, the expanded beads are dried in an oven at 60° C. for 24 hours.

After taking out the dried foamed particles from the oven, they are immediately left in a room set at 23° C. and a relative humidity of 50% for 72 hours. Then, in a room set under the same conditions, the mass of the expanded particles is obtained in the same manner as above. By subtracting the mass (F) of the expanded particles after cleaning from the mass (E) of the expanded particles before cleaning, which is obtained as described above, the amount (E)-(F) of the inorganic fine particles adhered to the surface of the expanded particles is subtracted. ) Is calculated. The amount (E)-(F) of the adhered inorganic fine particles is divided by the mass (E) of the expanded particles before washing, and then multiplied by 100 to convert into the mass per 100 g of the expanded particles. The amount of adhered fine particles can be determined.

From the viewpoint of suppressing the decrease in fusibility due to the above-mentioned inorganic fine particles and reducing the manufacturing cost and environmental load, etc., the amount of the dispersant used is reduced, or the expanded particles are produced without addition. Preferably. Specifically, in the foaming step, when inorganic fine particles are added as a dispersant, the addition amount thereof is preferably 0.5 parts by mass or less, and 0.3 parts by mass or less with respect to 100 parts by mass of the resin particles. Is more preferable, 0.1 mass part or less is more preferable, and no addition is particularly preferable.

From the same viewpoint, when a surfactant is added as a dispersant, the addition amount thereof is preferably 0.3 parts by mass or less, and 0.2 parts by mass or less based on 100 parts by mass of the resin particles. Is more preferable, 0.1 mass part or less is still more preferable, and it is particularly preferable not to add.

The expanded particles, the amount of the dispersant added to the dispersion is reduced, or even when the dispersant is not added to the dispersion, there is no blocking, the reason for good expanded particles, However, it is not completely clear, but the following reasons can be considered.

Since the coating layer contains, as the polymer type antistatic agent (C), a polyether-polyolefin block copolymer having a melting point satisfying a specific relationship with the melting point of the polyethylene-based resin of the core layer, foaming occurs. It is considered that an electrostatic repulsive force is generated between the resin particles in the process. In addition, the polyether-polyolefin block copolymer is excellent in compatibility with the polyethylene-based resin as compared with other polymer type antistatic agents, so that it is easily dispersed uniformly in the coating layer, and electrostatic repulsion It is expected that the size will be larger. As a result of these, it is considered that the expanded particles can suppress blocking even when the amount of the dispersant added to the dispersion is reduced or even when the dispersant is not added to the dispersion.

Examples of the expanded beads and the expanded beads molded article according to the present invention will be described. In this example, the materials shown in Tables 1 and 2 were used to produce the expanded particles (Examples 1 to 13) shown in Tables 3 to 4. Further, as comparative examples, expanded particles (Comparative Examples 1 to 18) shown in Tables 5 to 7 were produced. The resin density, MFR, melting point, and heat of fusion shown in Table 1 are values measured by the above-described methods. Further, the MFR and melting point of the polymeric antistatic agent shown in Table 2 are values measured by the above-mentioned method. However, the melt flow rate of PA/PE2 is a value measured under the conditions of a temperature of 230° C. and a load of 2.16 kg in accordance with JIS K7210-1:2014. As described above, "Perestat" and "Perectron" are registered trademarks of Sanyo Chemical Industries, Ltd.

The method for producing the expanded beads is specifically as follows.

-Examples 1-13 and comparative examples 6-18
(Extrusion/cutting process)
A core layer forming extruder having an inner diameter of 65 mm and a coating layer forming extruder having an inner diameter of 30 mm are provided side by side, and a multi-strand-strand-like die capable of coextruding is attached to the outlet side of the coextruder. To produce a strand.

For the core layer forming extruder, the resins shown in the "core layer" column of Tables 3, 4, 6 and 7 and the antistatic agent in the compounding amounts shown in the table are added to 100 parts by weight of the resin. 0.02 parts by mass of a bubble regulator was supplied and melt-kneaded in the extruder. In addition, "LL1+HD" in the table means 90% by mass of linear low-density polyethylene (LL1) and 10% by mass of high-density polyethylene (HD) (however, the total of linear low-density polyethylene and high-density polyethylene is 100%). Mass%) was supplied to the extruder and kneaded in the extruder. Also, as the bubble control agent, "Zinc borate 2335 (average particle size 6 μm)" manufactured by Tomita Pharmaceutical Co., Ltd. was used.

The coating layer forming extruder was supplied with the resins shown in Tables 3, 4, 6, and 7 and the antistatic agent in the compounding amounts shown in the table, and melt-kneaded in the extruder.

Then, the melt-kneaded product was co-extruded from each extruder so that the mass ratio of the coating layer to the mass of the entire extrudate would be the value shown in the "ratio" column of Tables 3 to 7. The melt-kneaded products extruded from each extruder are merged in a die, and are extruded in the form of a multi-layer strand in which the outer periphery of the core layer is covered with a coating layer from the pores of the die attached to the tip of the extruder. By cooling this coextrudate with water, a multi-layered strand was obtained.

The obtained strand was cut with a pelletizer so that the mass was about 3 mg to obtain a multilayer resin particle. The ratio of the length to the diameter of the resin particles (L/D) was 1.0.

(Foaming process)
1 kg of the resin particles and the amount of the dispersant shown in Table 3, Table 4, Table 6 and Table 7 with respect to 100 parts by mass of the resin particles were enclosed in a closed container together with 3 L of water as a dispersion medium. In this example, kaolin and/or sodium alkylbenzene sulfonate (LAS) was used as the dispersant.

Next, carbon dioxide was supplied as a foaming agent into the closed container to pressurize the inside of the container. Then, the inside of the container was heated with stirring to raise the inside of the container to the foaming temperature shown in Table 3, Table 4, Table 6 and Table 7. After maintaining this foaming temperature for 15 minutes, the airtight container was opened and the contents were discharged under atmospheric pressure to foam the resin particles. As described above, primary expanded particles having a sheath-core structure having a foamed core layer and a coating layer covering the core layer were obtained.

In this example, the dispersibility of the foamed particles was evaluated based on the presence or absence of fusion (so-called blocking) of resin particles in this foaming step. Specifically, after completion of the foaming step, it was visually evaluated whether or not the primary foamed particles taken out from the closed container contained a lump formed by fusion of the resin particles. In Table 3, Table 4, Table 6 and Table 7, in the "Dispersibility" column, it was determined that blocking did not occur when the above-mentioned lumps were not contained in the primary expanded particles, and the symbol "○" was described. did. Further, when the above-mentioned lumps were contained in the primary expanded beads, it was determined that blocking had occurred, and the symbol “x” was described in the same column.

In Comparative Examples 6, 13, 15, 17, and 18, the dispersibility of the resin particles in the closed container was poor, and the resin particles fused to each other in the container. Therefore, for these comparative examples, the subsequent steps and evaluations were not performed.

(Two-stage foaming process)
After performing the foaming step, a two-stage foaming step was performed to adjust the apparent density of the expanded particles. The two-step foaming step was specifically performed as follows. First, the primary expanded particles were impregnated with pressurized air in a pressure vessel, and the internal pressure of the expanded particles was adjusted to the values shown in Tables 3 to 7. After filling the primary foamed particles in a compact pressure foaming machine (J-080 manufactured by Daisen Kogyo Co., Ltd.), the primary foamed particles are heated by steam at a pressure (gauge pressure) shown in Table 3, Table 4, Table 6 and Table 7. To further foam. As a result, expanded particles (that is, secondary expanded particles) having the apparent densities shown in Tables 3, 4, 6, and 7 were obtained.

-Comparative examples 1-5
(Extrusion/cutting process)
The resin shown in Table 5, the antistatic agent having the compounding amount shown in the same table, and 0.02 part by mass of the above-mentioned bubble adjusting agent with respect to 100 parts by mass of the resin were supplied to the extruder, and melt-kneaded in the extruder.

The melt-kneaded product in the extruder was extruded into a strand shape and then water-cooled to obtain a single-layer strand. The obtained strand was water-cooled and cut with a pelletizer to a mass of about 1.3 mg to obtain a single layer of resin particles.

The obtained resin particles were subjected to a foaming step and a two-stage foaming step in the same manner as the multilayer resin particles to obtain expanded particles (secondary expanded particles). Also, the dispersibility in the foaming step was evaluated in the same manner as the multilayer resin particles. In Comparative Examples 1 and 4, the dispersibility of the resin particles in the closed container was poor, and the resin particles were fused to each other in the container. Therefore, for these comparative examples, the process after the foaming process and the evaluation were not performed.

(Molding process)
Next, a method for manufacturing a foamed particle molded body will be described. First, a mold having a flat plate-shaped cavity having a length of 250 mm, a width of 200 mm, and a thickness of 50 mm was prepared, and the foamed particles were filled in the cavity. Next, steam was introduced into the cavity so that the molding pressures (gauge pressures) shown in Tables 3 to 7 were introduced to heat the foamed particles, whereby the foamed particles were fused to each other to obtain a foamed particle molded body. .. After the expanded beads were sufficiently heated, the mold was cooled, and the expanded particle molded body was taken out from the mold. The foamed particle molded body thus obtained was allowed to stand in an oven adjusted to 80° C. for 12 hours to dry and cure the foamed particle molded body.

The following evaluations were carried out on the characteristics of the raw materials used for the expanded beads of the examples and comparative examples. Regarding the single-layer expanded beads, the entire expanded particles are treated as the core layer.

・Melt flow rate of core layer (MFR(I))
The core layer extruded from the core layer forming extruder was used for the multilayer resin particles, and the strand extruded from the extruder was used for the single layer resin particles. Using these as measurement samples, MFR(I) was measured under the conditions of a temperature of 190° C. and a load of 2.16 kg in accordance with JIS K7210-1:2014.

・Melt flow rate of coating layer (MFR(II))
The MFR(II) of the coating layer extruded from the coating layer forming extruder was measured under the conditions of a temperature of 190° C. and a load of 2.16 kg in accordance with JIS K7210-1:2014.

・Melting point Tm _(A) of resin ( _A)
As described above, the melting point Tm _(A) of the resin _(A) was measured by the method according to JIS K7121:2012. Specifically, using a heat flux differential scanning calorimeter (manufactured by SII Nano Technology Inc., model number: DSC7020), the resin (A) was heated from 23°C to 200°C at a heating rate of 10°C/min. A DSC curve was obtained by heating, then cooling from 200° C. to 30° C. at a cooling rate of 10° C./min, and then heating from 30° C. to 200° C. at a heating rate of 10° C./min. The apex temperature of the melting peak appearing in this DSC curve was defined as the melting point Tm _(A) of the resin (A). When a plurality of melting peaks appeared on the DSC curve, the peak temperature of the melting peak having the largest area was taken as the melting point Tm _(A) . When the resin (A) is a mixed resin, the resin was melt-kneaded in advance and pelletized mixed resin particles were used as a measurement sample.

・Melting point Tm _(B) of resin ( _B)
It was measured melting point of the resin of _(B) Tm (B) measured in the same manner as the melting point Tm _(A) of the above-mentioned resin (A).

-Amount of heat of fusion of resin (A) The amount of heat of fusion of resin (A) was measured by the method based on JIS K7122:2012. Specifically, using a heat flux differential scanning calorimeter (manufactured by SII Nano Technology Inc., model number: DSC7020), the resin (A) was heated from 23°C to 200°C at a heating rate of 10°C/min. A DSC curve was obtained by heating, then cooling from 200° C. to 30° C. at a cooling rate of 10° C./min, and then heating from 30° C. to 200° C. at a heating rate of 10° C./min. The area of the melting peak in this DSC curve was taken as the heat of fusion. When a plurality of melting peaks appeared on the DSC curve, the total area of the plurality of melting peaks was taken as the heat of fusion. When the resin (A) is a mixed resin, the resin was melt-kneaded in advance and pelletized mixed resin particles were used as a measurement sample.

-Amount of heat of fusion of resin (B) The melting point of resin (B) was measured by the same method as the method of measuring the amount of heat of fusion of resin (A) described above.

In addition, the following evaluations were performed on the expanded particles of the examples and comparative examples.

-High temperature peak calorific value The high temperature peak calorific value of the core layer of the expanded particles was measured by the method described above. That is, heat flux DSC was performed using about 3 mg of the expanded particles from which the coating layer was removed, and the peak area of the high temperature peak in the obtained DSC curve was taken as the high temperature peak calorie of the core layer of the expanded particles. The measurement start temperature in the heat flux DSC was 23° C., the measurement end temperature was 200° C., and the temperature rising rate was 10° C./min. A heat flux differential scanning calorimeter (manufactured by Hitachi High-Tech Science Co., Ltd., model number: DSC7020) was used as a measuring device.

-Apparent Density After precisely weighing the mass of the expanded particle group consisting of a large number of expanded particles, a graduated cylinder containing water was prepared, and the expanded particle group was completely submerged in water using a wire net. The volume of the expanded particle group was determined from the amount of rise of the liquid surface at this time. The apparent density of the expanded particles was calculated by dividing the mass of the expanded particles thus obtained by the volume. The apparent density of the expanded particles was as shown in the "Apparent Density" column of Tables 3 to 7. As the expanded particles to be measured, either the primary expanded particles or the secondary expanded particles were used.

-Amount of inorganic fine particles deposited The amount of inorganic fine particles deposited was calculated by the method described above. That is, by using about 100 g of the primary expanded particles, the mass (E) of the expanded particles to which the inorganic fine particles were attached and the mass (F) of the expanded particles after washing the inorganic fine particles were measured by the above-mentioned method. The amount (E)-(F) of the inorganic fine particles attached to the expanded particles calculated from these values is divided by the mass (E) of the expanded particles to which the inorganic fine particles are attached, and then multiplied by 100 to obtain 100 g of expanded particles. The amount of the inorganic fine particles adhering to 100 g of the expanded particles was calculated by converting the mass into

Next, the following evaluations were carried out on the expanded-particle molded products produced from the expanded particles of Examples and Comparative Examples.

Moldability of Expanded Particles The moldability of expanded particles was specifically evaluated based on the fusion rate, surface properties, and recoverability. The evaluation method of each evaluation item is as follows.

(Fusing rate)
The foamed particle molded body was bent and broken so as to be divided into substantially equal parts in the longitudinal direction. The exposed fractured surface was visually observed, and the number of foamed particles in which the interface between the foamed particles was peeled off and the number of foamed particles broken inside were counted. Then, the total number of the foamed particles exposed on the fracture surface, that is, the number of foamed particles in which the interface between the foamed particles is peeled off, and the inside of the foamed particles with respect to the total of the number of foamed particles broken inside The ratio of the number of foamed particles was calculated. A value obtained by expressing this ratio in percentage (%) was defined as a fusion rate.

(Surface texture)
A 100 mm×100 mm rectangle was drawn in the center of the expanded particle molded body, and then a diagonal line was drawn from any corner of this rectangle. The number of voids (that is, gaps between the expanded particles) formed so as to overlap the diagonal line and larger than a square having a side of 1 mm was counted. In the “surface texture” column of Tables 3 to 6, when the number of voids was less than 20, it was judged that the surface texture was good, and the symbol “◯” was described. Further, when the number of voids was 20 or more, it was determined that the surface properties were inferior, and the symbol "x" was described in the same column.

-Recoverability The presence or absence of sink marks in the foamed particle molded body, that is, the state in which the center of the molded body is more depressed than the surroundings, was evaluated. Specifically, the thicknesses of the central portion and the four corner portions of the obtained expanded particle molded article were measured, and the ratio of the thickness of the central portion to the thickest portion of the four corner portions was calculated. In the “Recoverability” column of Tables 3 to 6, when the thickness ratio is 90% or more, it is judged that the sink is not generated or the sink is sufficiently small and the recoverability is good. "○" is described. Further, when the thickness ratio is less than 90%, it was judged that the recovery was inferior because the sink mark was large, and the symbol "x" was described in the same column.

-Molded Body Density The foamed particle molded body was allowed to stand for 48 hours in an environment of a temperature of 23°C and a relative humidity of 50% RH, and then the mass of the foamed particle molded body was precisely weighed. A container containing water was prepared, and the expanded particle molded body was completely submerged in water using a wire net. The volume of the expanded particle molded body was determined from the amount of rise of the liquid surface at this time. The apparent density of the expanded particles was calculated by dividing the mass of the expanded particle molded product thus obtained by the volume.

-Surface resistivity The surface resistivity of the expanded particle molded article was measured by a method based on JIS K 6271-1:2015. Specifically, first, the foamed particle molded body is allowed to stand for one day in an environment of a temperature of 23° C. and 50% RH for curing, and then, from the center of the foamed particle molded body, a length of 100 mm×a width of 100 mm×a thickness of 10 mm. A rectangular parallelepiped test piece was cut out. At this time, one of the two surfaces of 100 mm in length×100 mm in width present in the rectangular parallelepiped is the skin surface, that is, the surface of the foamed particle molded body obtained by in-mold molding, and the other is the inner surface, that is, the foamed particles. The test piece was cut out so that the inside of the molded body was the exposed surface. Further, the test piece was manufactured by the same method in an environment of a temperature of 23° C. and 12% RH.

After that, the surface resistivity was measured using a resistivity meter (“HIRESTA MCP-HT450” manufactured by Mitsubishi Chemical Analytech Co., Ltd.) at five measurement positions randomly selected from the skin surface of the test piece. "URS100" manufactured by Mitsubishi Chemical Analytech Co., Ltd. was used as the probe. The temperature of the measurement environment was 23° C., the relative humidity was 50% RH, the applied voltage during measurement was 500 V, and the application time was 30 seconds. The surface resistivity was measured by the same method in an environment of temperature 23° C. and 12% RH. When the surface resistivity exceeded 1×10 ¹³ , it was displayed as “>10 ¹³ ”.

-Migration of Antistatic Agent A rectangular parallelepiped test piece having a length of 180 mm × a width of 180 mm × a thickness of 20 mm was cut out from the vicinity of the center of the expanded particle molded body. At this time, one of the two surfaces of 180 mm in length×180 mm in width present in the rectangular parallelepiped is the skin surface, that is, the surface of the expanded particle molded body obtained by in-mold molding, and the other is the internal surface, that is, the expanded particles. The test piece was cut out so that the inside of the molded body was the exposed surface.

Then, 10 slide glasses used for the test were piled up to make a blank sample, and the haze value of the blank sample was measured. The haze value was measured using a haze meter (NDH5000) manufactured by Nippon Denshoku Industries Co., Ltd. Then, 10 slide glasses whose haze values were measured were arranged side by side on the skin surface of the test piece, and one surface of each slide glass was brought into contact with the skin surface. Then, another test piece was placed on the other surface of the glass slide so that the skin surface was in contact with the other surface of the glass slide.

After sandwiching 10 slide glasses between the skin surfaces of the test pieces in this manner, a weight was placed on the test pieces arranged above, and a load of 1 kg/100 cm ² applied to the slide glasses was applied. In this state, the test piece and the slide glass were heated for 72 hours in an oven at 60° C. and a relative humidity of 60%. After measuring the haze value after heating by stacking 10 slide glasses after heating, increase the haze value by subtracting the haze value of the blank sample (that is, the slide glass before heating) from the haze value after heating. Calculated. In Tables 3 to 7, the "migration" column shows the symbol "○" when the increase in haze value is less than 10, the symbol "△" when 10 or more and less than 15, and the symbol 15 when it is 15 or more. The symbol "x" is described.

As shown in Table 3, in Examples 1 to 8, polyethylene-based resin (A) containing linear low-density polyethylene in the core layer and polyethylene-based resin (B) containing linear low-density polyethylene in the coating layer were used. And a polymer type antistatic agent (C) composed of a polyether-polyolefin block copolymer. Then, the melting point Tm _{(A) of} the polyethylene resin (A), the melting point Tm _{(B) of} the polyethylene resin (B) and the melting point Tm _(C) of the polymer type antistatic agent _(C) are expressed by the above-mentioned formula (1). ~ Expression (2) is satisfied. Therefore, the expanded beads of the examples had excellent expandability and moldability. Further, the expanded-particle molded products produced from the expanded particles of the examples had excellent antistatic performance even in an environment of low humidity, and were able to suppress migration of the antistatic agent.

The expanded particles of Examples 9 to 13 shown in Table 4 are examples produced by reducing the amount of the dispersant added to the dispersion or adding no dispersant to the dispersion in the production process. As shown in Examples 9 to 13, the expanded particles having the specific core layer and the coating layer have a reduced amount of the dispersant, or even when produced without adding the dispersant to the dispersion liquid, It was possible to produce expanded particles having good moldability without blocking and the like. Further, the expanded particles of Examples 9 to 13 have excellent antistatic performance even in an environment of low humidity and can suppress migration of the antistatic agent, as in Examples 1 to 8. It was

Comparative Example 1 is an example in which a single-layer expanded particle containing a polyethylene resin and a surfactant as an antistatic agent was prepared. In Comparative Example 1, the resin particles could not be dispersed in the dispersion medium.

Comparative Example 2 is an example in which the same expanded particles as in Comparative Example 1 were produced using a dispersant. The expanded-particle molded product including the expanded particles of Comparative Example 2 cannot suppress migration of the surfactant from its surface to the object to be packed, and may cause contamination of the object to be packed.

Comparative Example 3 is an example of a single-layer expanded particle containing a polyethylene resin and a polymer type antistatic agent. In the case of the single-layer expanded beads, when the amount of antistatic performance required for exhibiting the desired antistatic performance is blended, the fusible properties and the recoverability of the expanded particles deteriorate.

Comparative Example 4 is an example in which the content of the antistatic agent is smaller than that in Comparative Example 3 and no dispersant is used. In Comparative Example 4, the resin particles could not be dispersed in the dispersion medium.

Comparative Example 5 is an example in which the same expanded particles as in Comparative Example 4 were produced using a dispersant. As shown in Comparative Example 5, when the polymer type antistatic agent is used in the single-layer expanded beads, the antistatic performance becomes insufficient when the content is the same as that of the coating layer in the multi-layer expanded particles.

Comparative Example 6 is an example in which a multilayer foamed particle in which a surfactant as an antistatic agent was mixed in the coating layer was prepared without using a dispersant. In the expanded beads of Comparative Example 6, the resin particles could not be dispersed in the dispersion medium.

Comparative Example 7 is an example in which the same expanded particles as in Comparative Example 6 were produced using a dispersant. As shown in Comparative Example 7, when a surfactant is used as the antistatic agent, migration of the surfactant from the surface of the expanded particles to the object to be packed can be suppressed even in the case of expanded particles having a plurality of layers. However, there is a risk that the object to be packed may be contaminated.

In Comparative Example 8, the content of the polymeric antistatic agent (C) was less than the specific range, and therefore the antistatic performance was insufficient.

In Comparative Example 9 and Comparative Example 16, the melting point Tm _(A) of the polyethylene resin (A) contained in the core layer was lower than the melting point Tm _{(B) of the} polyethylene resin (B) contained in the coating layer. Therefore, the fusion rate of the expanded particles was significantly reduced. In addition, the recoverability of the foamed particle molded product deteriorated, and sink marks were generated in the molded product after curing.

Comparative Examples 10 to 12 are examples in which the type of the polymeric antistatic agent (C) was changed from Example 3, and the polyethylene resin (A) and the polymeric antistatic agent (C) in these Comparative Examples were used. The difference Tm _(C) -Tm _(A) between the melting points and Therefore, the fusibility of the expanded beads was reduced.

Comparative Examples 13 and 15 are examples in which the type of the polymeric antistatic agent (C) was changed from Example 1. In these comparative examples, since the polyether-polyolefin block copolymer was not used as the polymer type antistatic agent (C), the resin particles could not be dispersed in the dispersion medium and blocking occurred.

Comparative Example 14 is an example in which the type of the polymeric antistatic agent (C) was changed from that of Example 3. Since no polyether-polyolefin block copolymer was used as the polymer type antistatic agent (C), the antistatic performance was insufficient.

Comparative Example 17 is an example of expanded particles in which the core layer and the coating layer are made of polypropylene resin. With respect to the expanded particles made of polypropylene resin, the resin particles could not be dispersed in the dispersion medium unless a dispersant was added, and blocking occurred. Since the polypropylene-based resin foamed particles are foamed at a foaming temperature higher than the melting point of the resin, the resin is significantly softened in the foaming process, and when no dispersant is added, mutual adhesion of the resin particles can be suppressed. Probably difficult.

Comparative Example 18 is an example in which the same expanded particles as in Comparative Example 17 were produced using a small amount of a dispersant. In the case of expanded particles having a core layer and a coating layer composed of a polypropylene resin as in Comparative Example 18, when the amount of the dispersant added to the dispersion is small, the resin particles should be dispersed in the dispersion medium. And blocking occurred.

Claims (7)

A core layer made of a foam containing a polyethylene-based resin (A),
A polyethylene resin (B) and a polymeric antistatic agent (C), and a coating layer covering the core layer,
The polyethylene resin (A) contains linear low-density polyethylene,
The polyethylene resin (B) contains linear low-density polyethylene,
The polymer type antistatic agent (C) contains a polyether-polyolefin block copolymer,
The content of the polymeric antistatic agent (C) in the coating layer is 20 to 50% by mass,
The melting point Tm _(A) [℃] of the polyethylene resin (A), the melting point Tm _(B) [℃] of the polyethylene resin (B), the melting point Tm of the polymer type antistatic agent (C) _{( C)} [℃]
0≦Tm _(A) −Tm _(B) ≦30 (1)
25≦Tm _(C) −Tm _(A) ≦75 (2)
Polyethylene-based resin expanded particles that satisfy the above relationship. The polyethylene-based resin (A) melting point Tm _(A) of [° C.], a melting point Tm of the polymer type antistatic agent _{(C) (C) [℃} ] and although,
30≦Tm _(C) −Tm _(A) ≦70 (2-1)
The expanded polyethylene resin particles according to claim 1, satisfying the relationship of. The expanded polyethylene resin particles according to claim 1 or 2, wherein the amount of the inorganic fine particles adhered to the surface of the expanded polyethylene resin particles of 100 g of the expanded polyethylene resin particles is 200 mg or less. The polyethylene melting point Tm _(A) [° C.] of the resin (A) and the melting point Tm _(B) [° C.] of the polyethylene resin (B) is higher than both 100 ° C., any one of claims 1 to 3 The expanded polyethylene resin particles according to item 1. The polyethylene resin expanded particles according to any one of claims 1 to 4, wherein the polyethylene resin (A) is a mixture containing linear low-density polyethylene and high-density polyethylene. The melt flow rate value (I) of the core layer obtained by measuring at a temperature of 190° C. and a load of 2.16 kgf is 0.5 to 5 g/10 min, which is equal to the melt flow rate value (I) of the core layer. The value of ratio (II)/(I) with respect to the value (II) of melt flow rate of the coating layer obtained by measurement at a temperature of 190° C. and a load of 2.16 kgf is 1 to 5. 5. The polyethylene resin expanded particles according to any one of 5 above. A polyethylene-based resin expanded-particle molded article obtained by in-mold molding of the polyethylene-based resin expanded particles according to any one of claims 1 to 6,
A polyethylene resin foamed particle molded product having a surface resistivity of less than 1×10 ¹³ Ω at a relative humidity of 12% RH.

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